Process of and apparatus for separating gas mixtures



Patented Aug. l, 1950 PROCESS OF AND APPARATUS FOR SEPARATING GAS MIXTURES Philip K. Rice, Kenmore, N. Y., assignor to The Linde Air Products Company, a corporation of Ohio Application'December 113, 1944, Serial No. 567,951

Claims. (Cl. 62-1755) This invention relates to a process of and 8.9- paratus for separating gas mixtures, and more particularly to a process of and apparatus for separating air to recover oxygen, especially oxygen in the liquid state.

Oxygen either in the gaseous or liquid state is customarily produced in large stationary plants by a process requiring the compression of the air to high pressures such as 2000 to 3000 p. s. i. Such pressures require the use of four stages or more of compression and the compressors and intercoolers employed are necessarily large and heavy. The other high pressure equipment such as traps, heat exchangers, and expansion engines are likewise of heavy construction. With air at such high pressures, there is also considerable difficulty in maintaining the apparatus free of leaks.

Air separation cycles operating at low pressures have been proposed. For example, a compression pressure of 70 p. s. i. would permit the use Of a multi-stage turbo compressor having a relatively moderate weight for the quantity of air processed. Such low pressure cycles however require that a considerably greater quantity of air must be compressed and expanded solely to produce the refrigeration needed, and the amount of such air is substantially greater than the air which is separated to produce the liquid oxygen. The air at such low pressures has a very large volume and therefore the heat exchangers and air handling equipment must be bulky. The compressors and expansion engines must be correspondingly larger to handle the excess air which must be expanded to produce the required refrigeration. The use of such large volumes of air in heat exchangers involves greater heat loss in the exchangers so that the actual efficiency is much less than that which would theoretically .be expected.

Objects of the present invention therefore are to provide a process of and apparatus for separating a mixture of gaseshaving low boiling points such as air: which avoids the difficulties pointed out hereinabove; which requires the compression of the gas mixture only to an intermediate pressure of between about 150 to 400 p. s. i. gauge; which employs compression and gas handling equipment of moderate size and relatively light weight; which avoids the use of auxiliary devices for removing such solidifiable impurities as carbon dioxide from the'gas mixture before compression or before cooling the mixture to a low temperature; and which can be conveniently employed in a portable plantfor the production of a liquefied gas such as liquid oxygen.

These and other objects and advantages of this invention will become apparent from the following description and the accompanying drawing, in which:

Fig. 1 is a diagrammatic view showing a preferred embodiment of apparatus for carrying out the process of the invention, particularly for separation of air to produce liquid oxygen; and

Fig. 2 is a diagrammatic fragmentary view showing a modification of the apparatus.

According to a preferred embodiment of the invention which will be described for the separation of air to produce liquid oxygen, air in larger amount than the amount of air to be separated is compressed by a lightweight, high-speed compressor to a pressure between and 400 p. s. i. and preferably to a pressure of about 270 p. s. i. gauge without any preliminary treatment to remove carbon dioxide. The moisture in the air is removed substantially completely and preferably by refrigeration of the air to a relatively low temperature, for example a temperature of -45 to -50 C. The dried air is then further cooled by heat exchange with outgoing products to a lower temperature which is preferably about -l05 C. The air is then divided into two portions, one of which is expanded with the production of external work in an expansion engine to cool it substantially to its condensation temperature. The cold expanded air is added to the outflowing nitrogen product of separation which passes through a countercurrent heat exchanger for countercurrently cooling the remainder of the compressed air. The cooled remainder of the compressed air is then passed through a heat exchanger in the base of a rectifying column to supply heat to the lower end of the column and become itself substantially all liquefied. The thus liquefied remainder of the air still contains the impurities such as carbon dioxide in a finely divided solid form. The liquefied remainder is expanded to a pressure close to the relatively low pressure of rectification, after which such liquid is passed through filters which remove the solidified impurities including carbon dioxide. The filtered liquid is then passed into a rectifying column for separation, the liquid oxygen product collecting in and being removed from the lower part of the column and a nitrogen product passing from the upper end of the column.

By operating at the intermediate pressure of about 270 p. s. i., a relatively lightweight high speed reciprocating compressor having only two or three compression stages maybe employed. The compressed gas handling equipment need not be very heavy nor need it be of large volume. At this pressure little trouble with leaks. at rod packings is encountered. The thorough removal of water vapor prior to deep cooling and expansion avoids difficulties due to clogging of heat exchangers by solid carbon dioxide. .The provision of all the required refrigeration by engine expansion of a portion of the total air volume compressed, avoids the need of auxiliary refrigerating equipment which would include a separate compressor and condenser.

Referring now to the drawings and particularly to Fig. 1, a compressor is diagrammatically indicated at I0. This has an air inlet M, an interstage cooler I2, and an after-cooler I3. The discharge line I4 conducts the compressed air from.

the after-cooler to moisture removing devices I5 and I5, which are of the type that freeze out the moisture and are arranged in duplicate so that accumulated moisture in one can be thawed out while the other is in operation. The discharge line I4 is connected to either of the high pressure air passages I6 or IS in the moisture removers I5 and I5 through valves I1 and I1. The cold ends of the passages I6 and I6 are joined by conduits I8 and IE to a passage I9 within a heat exchanger 20, the conduits I8 and I8 being controlled by valves 2| and 2|. The cold end of passage I9 connects to a branch conduit 22 which conducts a part of the compressed and cooled air to an expansion device 23 preferably of the reciprocating type. The expansion engine may be of the type disclosed and claimed in my copending United States patent application Serial No. 569,971, filed December 27, 1944. The discharge of expanded air from the expansion device 23 is conducted by a conduit 24 to the lower end of a countercurrent heat exchanger 25. I

The remainder of the compressed air is conducted through the heat exchanger 25 by a passage 26, the inlet end of which is connected to the passage I9 and the discharge end of which is connected to a conduit 21 that conducts the cooled compressed remainder of the air to a heat-exchange'device or coil 28 located in the lower portion-of a rectifying column 29. From the heat exchange coil 28 the conduit 30 conducts the compressed remainder'to a pair of branch conduits 3| and 3| which connect to the inlet ends respectively of filters 32 and 32. The filters have their discharge ends connected to a conduit 33 by valves 34 and 34'. Conduits 33 conducts filtered remainder of the air to the upper portion of the rectifying column 29. Interposed in the branch conduits 3| and 3 I respectively are valves 35 and 35', the valves 34, 34 and 35, 35 are provided so that one of the two filters 32 or 32' may be in operation while the other is isolated for the purpose of removing the accumulated carbon dioxide and impurities. The remainder of the air is preferably expanded to a low pressure only slightly above the rectifying column pressure prior to passage through the filters. The valves 35 or 35 may be employed for the purpose of throttle expanding the liquid air. If desired however, the valves 35 and 35 may be employed solely as stop valves and a throttle valve 36 may be interposed in the conduit 30.

The rectifying column may be of a customary type suitable for the separation of air and is preferably a single-stage column in which the liquid oxygen accumulates in a sump 31 at the base of the column. Above the sump the column is provided with the customary type of gas and liquid contact devices, such as trays 33. A means for indicating the level of the liquid in the sump is diagrammatically indicated as a liquid level gauge 39. A valved conduit 40 is provided for drawing ofi the liquid oxygen from the sump 31.

The nitrogen product of separation leaves the upper end of the column 29 through a conduit 4| which conducts the cold nitrogen to the cold end of the heat exchanger to join with the discharge of the expansion engine. Conduit 4| preferably has interposed therein a control valve 4| for a purpose hereinafter described. The combined expanded air and nitrogen product leaving the warm end of exchanger 25, enters the heat exchanger 20 through a connection 42, and from the heat exchanger 20 the nitrogen-air mixture passes to a two-way valve 43 arranged to divert the mixture to either conduit 44 or 44' for discharge into the cold end of one of the moisture removing heat exchangers I5 and II respectively.

The warmed nitrogen-air mixture leaves the moisture remover I5 or I5 through conduit 45 or 45. v

For the purpose of heating the moisture remover passages I6 and I6 to thaw the accumulated moisture, there are provided valve controlled connections 46 and 46', at the lower ends of the removers I5 and I5 which may be used to pass a warm gas such as warm nitrogen-air mixture from one of the moisture removers through the other moisture remover. To drain off the thawed moisture accumulation, drain valves 41 and 41 are provided at the lower ends of the pasages I6 and I5 respectively.

The operation of the apparatus illustrated in Fig. l is as follows:

The air compressed by the compressor III to a presure of about 270 p. s. i. is cooled atmospherically or by cooling water in the after-cooler l3 and then is cooled to a temperature between 45 and 50 C. in the moisture removing coolers I5 and I5. The moisture accumulates as frost in the cooling passages IE or IE, according to which moisture remover is in operation. If remover I5 is in operation for a certain length of time, moisture will have collected in the passages I6 until a pressure gauge indicates that an accumulation of moisture has occurred. The valves I1 and 2| will be closed, the valves I1 and 2| opened, the valve 43 shifted into the position illustrated in the drawing to send the nitrogenair mixture through the moisture remover I5. Valve 45 will then be opened and warm gas passed through the remover I5 to thaw out the passage I6 and the valve 41 will be opened to drain away the accumulated moisture.

In the heat exchanger 20 the dried compressed air is cooled to a temperature of about -l05 C. About a little over half of the air is passed to the expansion engine 23 and there expanded from the pressure of 2'70 p. s. i. to a pressure of a few p. s. i. above atmospheric. Such expansion cools the air to a temperature of about C. The remainder of the compressed air is cooled in the countercurrent heat exchanger 25 to its condensation, temperature and partially condensed. It is then totally condensed in the heat-exchange coil 28 and subcooled. The compressed air in the coil 28 thus supplies heat to the column'for the rectification. The liquefied air expands through the throttle valve 36 to a pressure which is only slightly above the pressure of the rectifying column 29 its temperature being correspondingly reduced. Such expanded air is filtered in one of the two filters if or 32' and passed to the upper end of the column for rectification therein. A separation product of substantially pure liquid oxygen accumulates in the sump 31 and may be drawn of! as required through the valved conduit 40; the liquid level gauge 39 being employed to maintain a sufllcient bath of liquid about the heat-exchanger coil 28. The cold nitrogen which leaves the column through the conduit 4| joins with the cold air from the expansion engine 23 and successively cools compressed air passing through the heat exchangers 25, 20 and one of the moisture removers 15 or l.

When it is desired to discharge the liquid oxygen product from connection 40 to a; receiver at a pressure slightly above atmospheric pressure, the control valve ll may be regulated to raise the column pressure without affecting the discharge pressure of the expansion engine. The valve 4i may be hand controlled or it may be an automatic back pressure valve responsive to its inlet pressure.

Referring to Fig. 2 ,a modified form of the apparatus employs a separator 50 interposed in the conduit ll which conducts the cooled remainder of the air from the heat exchanger 25. The separator 5t separates liquid and gaseous fractions of the cooled compressed remainder of the air and the liquid fraction which will contain substantially all of the impurities is passed through a conduit lit to the branch conduits 3i and 3| oi the filters it, 32'. From the filters, the filtered liquid fraction is passed to the upper portion of the rectifying column M9. The gaseous fraction from the separator 5t passes through a conduit hi to a heat-exchange device 52 in the lower portion of the column ma The heat-exchange device 52 is in the form of a multi-tube condenser in the sump ill of the column M9. The bottom end of the column serves as a collecting chamber M for the condenser 52 and the liquid collected therein is transferred by a conduit 54 controlled by an expansion valve 55 and conducting the expanded liquid to the upper. end of the column are.

The operation of the form of apparatus of Fig.

may be employed without others all within the spirit and scope of the invention. For example, while a single-stage rectifying column is described, a two-stage rectifying column of substantially the customary type may be substituted,

particularly in place of the column I29 in Fig. 2.

What is claimed is:

1. Process for separating air to obtain liquid oxygen which comprises compressing, cooling, and drying air to eliminate substantially all the moisture therefrom; cooling the dried air under pressure to a temperature substantially above the condensation temperature of oxygen at the compression pressure; expanding a substantial part of the cooled air with the production of external work too. pressure only slightly above atmospheric; utilizing all of said expanded part for refrigerating and partially liquefying the remainder of the air under pressure and for cooling the compressed air; separating liquid and gas phases of said remainder; expanding, filtering and rectifying the liquid phaseof said remainder to produce liquid oxygen and a nitrogen containing product; subjecting the gas phase of said remainder to heat exchange with the liquid oxygen product of rectification to form a liquid rich in nitrogen; utilizing the latter liquid as reflux in the rectification; and utilizing said nitrogen product for counter-currently cooling compressed air.

2. In an apparatus for separating a gas mixture by low temperature rectification, a heat exchanger for further cooling compressed and dried gas mixture; an expander for expanding one part of the cooled mixture with production of external work directly to nearly atmospheric pressure; a heat exchanger for effecting heat ex- 2 is quite similar to that of the apparatus of Fig. l

1 except thatthe cooled and partly liquefied remainder of the air is separated in the separator tit into liquid and gas fractions and only the liquid fraction is passed through the filters it or it to remove the solidified impurities before the material is passed into the rectifying column. The gaseous fraction of the still compressed remainder is condensed in the condenser 52 by heat exchange with the oxygen product collecting in the sump ill and is then expanded in the valve it and passed to the upper end of the column lit to form the reflux liquid therefor.

It will be seen that the invention provides a gas separation plant requiring only one compressor which may be relatively small and of lightweight construction, and utilizing gas handling equipment that may be of relatively light weight and also compact. The plant will deliver a liquefled gas product at a regulatable desired pressure above atmospheric pressure. Obviously all low temperature parts will be suitably protected by heat insulation and other various customary auxiliary devices, such as safety valves, pressure gauges, test connections and the like are also provided.

Although preferred embodiments of the invention have been described in detail, it is contemplated that modifications of the process and apparatus may be made and that some features change between the remainder of the mixture and all of such-expanded part; means for separating liquid and gas phases of said cooled remainder so that impurities are contained in the liquid phase; a filter device arranged to remove solid impurities from such liquid phase at reduced pressure; a rectifying column for receiving and rectifying said liquid phase to produce liquid oxygen and a nitrogen-rich product; heat exchanger means for subjecting the gas phase of said remainder to heat exchange with liquid oxygen product of rectification to form a reflux liquid; means for passing the latter liquid to the top of the rectifying column for use as reflux in the rectification; and means for utilizing said nitrogen-rich product to cool compressed air.

3. Process for separating a gas mixture by low temperature rectification to obtain a higher boiling component and a lower boiling gaseous component which comprises compressing said mixture to a pressure between about to 400 p. s. i.; removing substantially all the moisture in said mixture; cooling said dried mixture to a relatively low temperature substantially above the condensation temperature of any of the main components of said mixture; dividing said cooled mixture into two parts; expanding one part with the production of external work to the pressure of the outgoing separated gaseous component; countercurrently cooling the other of said parts by heat exchange with all of the expanded part; separating said other part into gas and liquid fractions under pressure; expanding said liquid fraction to substantially the pressure of rectification; filtering said expanded liquid fraction to remove solidified impurities; effecting heat exchange between said gas fraction and said separated higher boiling component to form a re tered liquid fraction and the expanded reflux liquid to produce'said higher boiling and said lower boiling components.

4. Process for separating a gas mixture by low temperature rectification to obtain a higher boiling component and a lower boiling gaseous component which comprises compressing said mixture to a pressure between about 150 to 400 p, s. 1.; removing substantially all the moisture in said mixture; cooling said dried mixture to a relatively low temperature substantially above the condensation temperature of any of the main components of said mixture; dividing said cooled mixture into two parts; expanding one part with the production of external work in a single stage directly to substantially atmospheric pressure; countercurrently cooling the'other of said parts by heat exchange with all of the expanded part; expanding said other part to a rectification pressure above the pressure to which said one part is expanded; rectifying the expanded other part at a regulated desired pressure above atmospheric to separate a component product having a higher boiling point from said lower boiling component; expanding said separated lower boiling component to the pressure to which said one part is expanded; and utilizing such expanded component for countercurrently cooling incoming compressed mixture.

5. Process for separating air which comprises compressing air to a pressure between about 150 to 400 p. s. i.; removing substantially all the moisture carried by said compressed air; cooling the dried compressed air to a temperature of about 105 C.; expanding one part of the cooled air with the production of external work in a single stage to a pressure slightly above atmos pheric; cooling the remainder of said air under said pressure by countercurrent heat exchange with all of the expanded part of the air; effecting separation of said cooled remainder into liquid and gas fractions; cooling and liquefying the gas fraction by heat exchange with liquid oxygen product; expanding" and rectifying the resulting liquid at a pressure not less than the pressure to which said one part is expanded; ex-

REFERENCES CITED The following references are of record in the file of this patent;

UNITED STATES PATENTS Number Name Date 1,626,345 Le Rouge Apr. 26, 1927 2,256,421 Borchardt Sept. 16, 1941 2,337,474 Kornemann et a1. Dec. 21, 1943 

